This invention relates in general to a magnetic testing apparatus more specifically it relates to a self controlled magnetic stray field threshold tester which gives warning signals when the external magnetic stray-field of an air freight consignement exceeds a predetermined value in a predetermined distance from the surface of air freight packages containing magnetized material. With air traffic safety involved such an apparatus must include means for self controlling its functioning.
Previously navigation in air traffic was mainly performed by the aid of a magnetic compass. Since air freight consignements containing magnetized materials may disturb the indication of the magnetic compass such shipments are classified as "Dangerous Goods". Although a magnetic compass is no longer of great importance in commercial air traffic, the maximum permitted distortion of the geomagnetic field in a distance of 15 ft (about 4.6 m) from any point of the surface of a consignement will not be changed. This maximum distortion should not exceed 0.418 A/cm -(0,0053 Oersted)-.
Measuring such a small stray field requires a hardware and personal expenditure that normally is beyond the means of the consignor and the carrier. Furthermore low-field measurements must be performed in an environment free from other magnetic disturbancies such as iron structures etc. In consequence consigments containing magnetized material quite often are not tested at all. On the other hand, the tonnage of magnetized materials, (i.a. strong permanent magnets), shipped by air has increased by more than a factor 100 during the past 15 years. Under those circumstances many shipments have been found with high external strayfields which even may disturb the function of other air plane equipment.
Thus there exists obviously an essential demand for a simple and inexpensive apparatus which enables also less experienced personnel in normal environment to determine quickly whether a package containing magnetized materialsatifies the above rules.